A non-directional hearing aid system allows a wearer to pickup sounds from any direction. When a hearing aid wearer is trying to carry on a conversation within a crowded room, a non-directional hearing aid system does not allow the wearer to easily differentiate between the voice of the person to whom the wearer is talking and background or crowd noise.
A directional hearing aid helps the wearer to hear the voice of the person with whom the wearer is talking, while reducing the miscellaneous crowd noise present within the room. One directional hearing aid system is implemented with a single microphone having inlets to cavities located in front and back of a diaphragm. An acoustic resistor placed across a hole in the back inlet of the microphone, in combination with the compliance formed by the volume of air behind the diaphragm, provides the single microphone with directionality. This directional hearing aid system is termed a first-order pressure gradient directional microphone. The term gradient refers to the differential pressure across the diaphragm. A first-order pressure gradient directional microphone relates to a microphone system that produces a signal based on the pressure differential across a single diaphragm.
One measure of the amount of directivity of a directional hearing aid system uses a polar directivity pattern, which shows the amount of pickup at a specific frequency (in terms of attenuation in dB) of a directional hearing aid system as a function of azimuth angle of sound incidence. A directivity index is the ratio of energy arriving from in front of the hearing aid wearer to the random energy incident from all directions around an imaginary sphere with the hearing aid at its center.
A first-order pressure gradient directional hearing aid microphone is capable of producing both a cardioid polar pattern and a super cardioid polar pattern. A cardioid polar pattern produces a directivity index of about 3-4 dB. A super cardioid polar pattern produces a directivity index of about 5-6 dB.
Persons with an unaidable unilateral hearing loss or persons having one ear that cannot be aided with a hearing aid (known as a dead ear) and one ear with some aidable hearing loss often have great difficulty communicating in high noise levels. These persons lose their auditory system's normal ability to suppress noise. With respect to a normal auditory system, the brain uses the balanced, fused, binaurally-processed inputs from the two normal cochleas of a normal hearing person, and cross-correlates these inputs to suppress noise.
Contralateral Routing Of Signals (CROS) and Bilateral Routing Of Signals (BI-CROS) hearing aids, respectively, are often employed for such persons since they often have great difficulty wearing only one hearing aid. CROS and BI-CROS system take sound from the bad ear, process it, then send the processed sound via hard wire, RF, or induction transmission to a receiver in the other ear.
CROS systems are used for individuals with on unaidable ear and one ear with normal hearing or a mild hearing loss. CROS systems includes a microphone and a receiver. A microphone is worn on the unaidable ear, and the receiver is worn on the better ear. BI-CROS systems are used for individuals having one unaidable ear and one ear needing amplification. BI-CROS systems include two microphones and a receiver. In the BI-CROS system, a microphone is worn on each ear, and the receiver is worn on the better ear. CROS and BI-CROS hearing aids overcome the loss of about 6 dB caused by the head blocking and diffracting sounds incident to one ear (the dead side) as they cross over to the better ear.
There is a need in the art to provide improved systems, devices and methods for providing hearing aid signals with more directionality to improve communications in high noise levels.